Controlled Mechanical Motions of Microparticles in Optical Tweezers

Inverted Sedimentation of Active Particles in Unbiased ac Fields

Gaining control over the motion of active particles is crucial for applications ranging from targeted cargo delivery to nanomedicine. While much progress has been made recently to control active motion based on external forces, flows, or gradients in concentration or light intensity, which all have a well-defined direction or bias, little is known about how to steer active particles in situations where no permanent bias can be realized. Here, we show that ac fields with a vanishing time average provide an alternative route to steering active particles. We exemplify this route for inertial active particles in a gravitational field, observing that a substantial fraction of them persistently travels in the upward direction upon switching on the ac field, resulting in an inverted sedimentation profile at the top wall of a confining container. Our results offer a generic control principle that could be used in the future to steer active motion, direct collective behaviors, and purify mixtures.

Spin Hall Effect of Double-Index Cylindrical Vector Beams in a Tight Focus

We investigate the spin angular momentum (SAM) of double-index cylindrical vector beams in tight focus. Such a set of beams is a generalization of the conventional cylindrical vector beams since the polarization order is different for the different transverse field components. Based on the Richards-Wolf theory, we obtain an expression for the SAM distribution and show that if the polarization orders are of different parity, then the spin Hall effect occurs in the tight focus, which is there are alternating areas with positive and negative spin angular momentum, despite linear polarization of the initial field. We also analyze the orbital angular momentum spectrum of all the components of the focused light field and determine the overwhelming angular harmonics. Neglecting the weak harmonics, we predict the SAM distribution and demonstrate the ability to generate the focal distribution where the areas with the positive and negative spin angular momentum reside on a ring and are alternating in pairs, or separated in different semicircles. Application areas of the obtained results are designing micromachines with optically driven elements.

Powering bioanalytical applications in biomedicine with light-responsive Janus micro-/nanomotors

Possessing both unique asymmetric structures and remote-controlled active movement, light-responsive Janus micro-/nanomotors offer the possibility of breaking through the limitations of traditional biomedicine, and have fascinated and inspired researchers. Despite many obstacles toward the clinical application, impressive progress of light-responsive Janus micro-/nanomotors for bioanalytical applications has been made over the past decades. In this review, we first briefly introduced several main light-driven Janus micro-/nanomotors, then focused on their typical bioanalytical applications such as biosensing, bioimaging, and theranostic. In the end, we summarized the remaining challenges of light-responsive Janus micro-/nanomotors in the practical application and also proposed potential solutions in the future.

Electrical Propulsion and Cargo Transport of Microbowl Shaped Janus Particles

Herein the effective electrical propulsion, cargo trapping, and transport capabilities of microbowl-shaped Janus particles (JPs) are demonstrated and evaluated. These active JPs are made by deposition of Au and Ti layers onto sacrificial spherical polystyrene particles, followed by oxidation of the Ti to TiO₂ . In contrast to the commonly studied spherical JP, the dual broken symmetry of both geometrical and electrical properties of the microbowl renders a strong dependence of its mobility and cargo loading on the order of the layering of Au and TiO₂ . Specifically, an opposite direction of motion is obtained for interchanged layers of Au and TiO₂ , using only electrical propulsion as the sole mechanism of motion. The concave side of the microbowl exhibits a negative dielectrophoretic trap of large size wherein trapped cargo is protected from hydrodynamic shearing, leading to an enhanced cargo loading capacity compared to that obtained using common spherical JP. Such enhanced cargo capability of the microbowl along with the ease of engineering it by interchanging the order of the layers are very attractive for future in vitro biological and biomedical applications.

Model systems for optical trapping: the physical basis and biological applications

The micromechanical methods, among which optical trapping and atomic force microscopy have a special place, are widespread currently in biology to study molecular interactions between different biological objects. Optical trapping is reported to be quite applicable to study the mechanical properties of surface structures onto bacterial (pili and flagella) and eukaryotic (filopodia) cells. The review briefly summarizes the physical basis of optical trapping, as well as the principles of calculating the van der Waals, electrostatic, and donor-acceptor forces when two microparticles or a microparticle and a flat surface are used. Three main types of model systems (abiotic, biotic, and mixed) used in trapping experiments are described, and the peculiarities of manipulation with living (bacteria, fungal spores, etc.) and non-spherical objects (e.g., rod-shaped bacteria) are summarized.

Single laser trapping for optical folding and rotation of red blood cells in sickle cell disease in response to hydroxyurea treatment

Optical folding and rotation behavior of red blood cells (RBCs) in polarized laser tweezers are considerably important for understanding the biophysical and biomechanical properties using the fast probing method. Here, a dual-mode polarized single-laser tweezers technique with distinct principal axes exhibiting different polarization states is presented and designed to investigate the deformation, optical folding, and rotation of single living cells with one measurement. RBC optical folding and rotation speed are measured in patients with sickle cell disease (SCD), including follow up of patients after hydroxyurea (HU) treatment for at least three months. Folding angle and rotation speed are significantly lower in patients with SCD and do not significantly differ in patients treated by HU compared with the healthy control group. The RBC folding angle and rotation speed in patients treated with HU drug increase linearly at lower laser powers and rapidly at higher powers, and increase much slowly in patients not treated with HU. The difference in the folding angle and rotation speed of RBCs could be useful for drug response in SCD or predicting pain crisis in SCD.

“Perfect” Terahertz Vortex Beams Formed Using Diffractive Axicons and Prospects for Excitation of Vortex Surface Plasmon Polaritons

Transformation of a Bessel beam by a lens results in the formation of a “perfect” vortex beam (PVB) in the focal plane of the lens. The PVB has a single-ring cross-section and carries an orbital angular momentum (OAM) equal to the OAM of the “parent” beam. PVBs have numerous applications based on the assumption of their ideal ring-type structure. For instance, we proposed using terahertz PVBs to excite vortex surface plasmon polaritons propagating along cylindrical conductors and the creation of plasmon multiplex communication lines in the future (Comput. Opt. 2019, 43, 992). Recently, we demonstrated the formation of PVBs in the terahertz range using a Bessel beam produced using a spiral binary silicon axicon (Phys. Rev. A 2017, 96, 023846). It was shown that, in that case, the PVB was not annular, but was split into nested spiral segments, which was obviously a consequence of the method of Bessel beam generation. The search for methods of producing perfect beams with characteristics approaching theoretically possible ones is a topical task. Since for the terahertz range, there are no devices like spatial modulators of light in the visible range, the main method for controlling the mode composition of beams is the use of diffractive optical elements. In this work, we investigated the characteristics of perfect beams, the parent beams being quasi-Bessel beams created by three types of diffractive phase axicons made of high-resistivity silicon: binary, kinoform, and “holographic”. The amplitude-phase distributions of the field in real perfect beams were calculated numerically in the approximation of the scalar diffraction theory. An analytical expression was obtained for the case of the binary axicon. It was shown that a distribution closest to an ideal vortex was obtained using a holographic axicon. The resulting distributions were compared with experimental and theoretical distributions of the evanescent field of a plasmon near the gold–zinc sulfide–air surface at different thicknesses of the dielectric layer, and recommendations for experiments were given.

Laser Induced Aggregation of Light Absorbing Particles by Marangoni Convection

Laser induced Marangoni convection can be used to accumulate micro-particles. In this paper, a method is developed to control and accumulate the light absorbing particles dispersed in a thin solution layer. The particles are irradiated by a focused laser beam. Due to the photothermal effect of the particles, the laser heating generates a thermal gradient and induces a convective flow around the laser’s heating center. The convective flow drives the particles to accumulate and form a particle aggregate close to the laser’s heating center. The motion of particles is dominated by the Marangoni convection. When the laser power is high, the vapor bubbles generated by laser heating on particles strengthen the convection, which accelerates the particles’ aggregation.

Plasmonic gold nanoparticles: Optical manipulation, imaging, drug delivery and therapy

Over the past two decades, the development of plasmonic nanoparticle (NPs), especially gold (Au) NPs, is being pursued more seriously in the medical fields such as imaging, drug delivery, and theranostic systems. However, there is no comprehensive review on the effect of the physical and chemical parameters of AuNPs on their plasmonic properties as well as the use of these unique characteristic in medical activities such as imaging and therapeutics. Therefore, in this literature the surface plasmon resonance (SPR) modeling of AuNPs was accurately captured toward precision medicine. Indeed, we investigated the importance of plasmonic properties of AuNPs in optical manipulation, imaging, drug delivery, and photothermal therapy (PTT) of cancerous cells based on their physicochemical properties. Finally, some challenges regarding the commercialization of AuNPs in future medicine such as, cytotoxicity, lack of standards for medical applications, high cost, and time-consuming process were discussed.